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There is QNX. seL4 is another.

The former is proprietary. The latter kernel is GPL2, similar to Linux.

QNX is not open source.

And seL4 is a kernel, not an OS. And it pretty hard to work with specially if you want any kind of dynamic system.

What did you mean by a dynamic system ?

One where processes, drivers and libraries come and go during the whole OS uptime.

There is Genode, which can run atop of seL4(amongst others).